In an etching process, a deposition process, an oxidation process or a sputtering process for manufacturing a semiconductor device or Flat Panel Display (FPD), plasma is widely used as a processing gas in order to facilitate a good reaction at a relatively low temperature. In such a type of plasma process, a high frequency (RF) power or microwave power is used to discharge or ionize the process gas within a vacuum processing container.
In a capacitively coupled plasma processing apparatus, an upper electrode and a lower electrode are disposed in parallel to each other within the processing container, the target substrate (e.g., a semiconductor wafer or a glass substrate) is placed on the lower electrode, and an RF power having a frequency (normally, 13.56 MHz or more) suitable for generating plasma is applied to the upper electrode or the lower electrode. Electrons are accelerated by the electric field generated between the electrodes facing each other due to an application of the RF power, and plasma is generated by the collision and ionization between electrons and the process gas. Also, a thin film may be deposited on a substrate, or a material or a thin film on the surface of the substrate may be etched due to the surface reaction by a gas phase reaction or a surface reaction of radicals or ions contained in the plasma.
As described above, radicals and ions incident onto the substrate play an important role in the plasma process. In particular, ions are important in that ions exhibit a physical action by an impact occurring when the ions are incident onto the substrate.
Conventionally, an RF bias method has been widely used in a plasma process. In the RF bias method, an RF power having a relatively low frequency (e.g., 13.56 MHz or less) is applied to a lower electrode, and ions contained in the plasma are accelerated and attracted on the substrate by a negative bias voltage or sheath voltage generated on the lower electrode. As a result, ions from plasma can be accelerated to be collided onto the substrate to facilitate surface reaction, anisotropic etching or film reforming.
[Patent Document 1]
Japanese Patent Application Laid-Open No. H7-302786
In the conventional capacitively coupled type plasma processing apparatus equipped with the RF bias function as described above, an RF for attracting ions is limited to one kind (single frequency), and the RF power and self-bias voltage or sheath voltage on the lower electrode are used as control parameters.
However, the present inventors have researched on a RF bias action in the course of developing a technology of a plasma process and have found out that a conventional method which uses a single RF for attracting ions has a difficulty in controlling the ion energy distribution in the state-of-the-art plasma process that requires a complex process characteristic.
More specifically, when analyzing the Ion Energy Distribution (IED) of ions that are incident on the substrate when a single RF is used for attracting ions, the energies of all incident ions are collected regularly in a continued energy band, and more ions are concentrated (a peak appears) in the vicinity of the maximum energy and the minimum energy, as illustrated in FIG. 15A to FIG. 15C and FIG. 16A to FIG. 16C. Accordingly, if an average of ion energies as well as the maximum energy and the minimum energy at which ions are more concentrated can be freely varied, an improvement of controllability of the RF bias function required for a plasma process is expected, but when the single RF is used, there is no case where the maximum energy and the minimum energy at which more ions are concentrated can be freely varied.
According to the conventional method, when an RF corresponding to a relatively low frequency of, for example, 0.8 MHz for attracting ions is used and the RF power is varied, the characteristic of ion energy distribution is changed as illustrated in FIG. 15A (low power level), FIG. 15B (intermediate power level), and FIG. 15C (high power level). That is, the maximum energy is varied into 1000 eV (FIG. 15A), 2000 eV (FIG. 15B), and 3000 eV (FIG. 15C) in proportional to the RF power while the minimum energy is being fixed at about 0 eV.
However, when an RF corresponding to a relatively high frequency of, for example, 13 MHz for attracting ions is used and the RF power is varied, the characteristic of ion energy distribution is changed as illustrated in FIG. 16A (low power level), FIG. 16B (intermediate power level), and FIG. 15C (high power level). That is, the maximum energy is varied into 650 eV, 1300 eV, 1950 eV in proportional to the RF power, while the minimum energy is also varied into 350 eV, 700 eV, 1050 eV in proportional to the RF power.
While FIG. 15A to FIG. 15C and FIG. 16A to FIG. 16C illustrate the characteristics of ion energy distribution of Ar+ ion, other ions exhibit the same characteristics (patterns) as well.
As described above, in the conventional method, even though the maximum energy or the average energy of the ion energy distribution may be arbitrarily varied, the minimum energy cannot be arbitrarily varied independently from the maximum energy. Therefore, the characteristic of the ion energy distribution, for example, indicated by an imaginary line (a dashed dotted line) K of FIG. 16C may not be achieved. Accordingly, a tradeoff between the etching rate, the selection ratio and the etched shape in a high aspect ratio contact (HARC) plasma etching may not be avoided readily.
The present disclosure has been made in an effort to solve the problems described above, and intends to provide a plasma processing apparatus which improves a controllability of the RF bias function, reliably prevents unwanted resonance from being generated on a RF transmission line between a counter electrode and ground potential, and enhances the reliability of the plasma process.